Vent structure

ABSTRACT

A vent structure for reliable long term operation at high temperatures is fabricated by annealing a tubing of predetermined length and size, uniformly filling a normally upper portion of the tubing with a predetermined thickness layer of fine alumina particles, flattening and doubly folding and pressing the upper portion of the tubing at predetermined pressures, annealing the formed tubing, and installing a particulate filter in the undeformed normally lower portion of the tubing. For use in venting helium generating reactor control pins located under hot molten sodium, a vent assembly including the vent structure and providing an air lock between the outer molten sodium and the vent is affixed to the upper end of each control pin.

O United States Patent 11 1 1111 3,838,557 McGuire Oct. 1, 1974 VENTSTRUCTURE 3,573,168 3/1971 Campbell 176/68 D 3,677,300 7/1972 King138/42 175] lnvemor- Joseph McGulre Kennewlck 3,697,377 10/1972 Gauthron176/68 x Wash.

[73] Assignee: McDonnell Douglas Corporation, Primary ExaminerFrank W.Lutter Santa Monica, Calif. Assistant ExaminerVincent Gifford Attorney,Agent, or Firm-D. N. Jeu; Walter J. Jason; [22] Filed. Sept. 4, 1973Donald L. Royer [21] App]. No.: 394,054

[57] ABSTRACT C1 A vent structure for reliable long term operation at 17176/ 6 R. 220/44 A high temperatures is fabricated by annealing a tubingCl l G216 B0101 3 of predetermined length and size, uniformly filling a1 Field of Search 176/68, 86 R; 5/ normally upper portion of the tubingwith a'predetermined thickness layer of fine alumina particles, flat-220/44 A tening and doubly folding and pressing the upper porv tion ofthe tubing at predetermined pressures, anneall Referelwes Cited ing theformed tubing, and installing a particulate fil- UNITED STATES PATENTSter in the undeformed normally lower portion of the 2,815,889 12 1957Stetz et al. 138/42 UX tubing- For use in Venting helium generatingreactor 2,992,659 7 1961 Thomas 138/42 Control P located under hotmolten Sodium a Vent 3,274,066 9/1966 Zumwalt I 17 assembly includingthe vent structure and providing an 3,321,285 5/1967 Sowman ...'176/68 Xair lockbetween the outer molten sodium and the 3,406,094 /19 8Beisswenger--.. 7 /68 X vent is affixed to the upper end of each controlpin. 3,459,636 8/1969 Germer 176/68 3,557,740 1/1971 Pratt 220/44 A xChums, 12 Drawmg Flglll'es ,4 //3 %*/6 -46 a s 32 k 1L g .24 Z? 26 M J!I /I l l l I ''/Z MEMEB 937 I summ Ta] W mm A VENT STRUCTURE BACKGROUNDOF THE INVENTION My present invention pertains generally to ventingdevices. More particularly, the invention relates to a very effectiveand practical vent structure for use in the control pins of a liquidmetal fast breeder reactor (LMFBR), for example, and to a novel methodof fabricating the vent structure.

As is well known, helium (He) is generated in the LMFBR control pins asthe result of a neutron, alpha (n, a) reaction on boron (B). This heliumis generated in each pin up to the rate of 3 cc/hr or 72 cc/day volumeat standard temperature and pressure (STP). If all of this generated gasis released from the boron carbide (8 C) matrix of a control pin, theresulting 26 liters per year poses a serious containment and pressureproblem Oct. 1 1, 1972 for Vent Structure and Method of Fabrication.While such disclosed vent structure and fabrication method are entirelysatisfactory, a vent structure providing more precise flowcharacteristics therethrough, and which structure can be moreconsistently I and easily fabricated to produce an accuratepredetermined flow, is desirable and useful in certain applications.

SUMMARY OF THE INVENTION Briefly, and in general terms, my invention ispreferably accomplished by providing a vent structure, including aductile and relatively thin wall tubing having an undeformed normallylower portion and a flattened and doubly folded and pressed normallyupper portion, in a molten sodium coolant reactor gas generating controlpin, for example, to release its generated gas normally at a relatvelylow flow rate such that the control pin (gas container) is maintained ina slightly pressurized state to prevent backflow of molten sodium or'vapor and consequential contamination thereof. The

formed (flattened and folded and pressed) upper portion of the ventstructure preferably includes a thin film or layer of fine aluminaparticles between the flattened opposing faces inside the tubing, toprevent the vent from sintering together ,due to grain growth across thefaces occurring at high temperatures and long term operation. The ventstructure preferably further includes a particulate filter in theundeformed lower portion of the vent tubing.

The method of fabricating the vent structure includes I the steps, amongothers, of applying a slight chamfer to the inside edge of the end of anormally upper portion of a metal tubing of predetermined length andsize, annealing the tubing of a predetermined temperature for apredetermined period, uniformly filling the upper portion of the tubingwith a predetermined thickness layer of fine alumina particles,flattening an upper portion of the tubing and then doubly folding andpressing such upper portion at predetermined pressures, and againannealing the tubing at a predetermined temperature for a predeterminedperiod. Finally, a porous metal particulate filter can be press-fittedinto the undeformed lower portion of the tubing. The resultant ventstructure is structurally stable and can operate reliably in a hightemperature environment over a long term, and gas flow thereof issubstantially a linear function of the applied gas pressure differentialup to the point where the elastic limit of the metal tubing is reached.

To fill the upper portion of the tubing with the uniform alumina layerprior to flattening, the tubing is placed with chamfered end downconcentrically about a polished control rod of predetermineddiameter andlength mounted upright on a flat resilient base. An appropriate amountof alumina particles is added at the top of the tubing which can besuitably held stationary against the surface of the resilient base, andthe alumina particles are packed down around the control rod using avibrator held against the side of the tubing. The base and rod are nextturned to horizontal and the central control rod is withdrawn to leave auniformly packed layer of alumina in the tubing. The normally upperportion of the tubing is then flattened, folded and pressed as describedabove.

For use in venting helium generating reactor control pins located undera corrosive and reactive liquid environment such as molten sodium, avent assembly including the vent structure and providing an air lockbetween the outer molten sodium and vent structure is attached to eachpin to ensure that liquid sodium does not directly contact the vent. Thevent assembly includes an adapter plug mounting the lower end of thevent structure, cover structure attached to the plug and forming an airlock chamber housing the part of the vent structure abovethe plug, and abaffle made of porous metal provided in the chamber space between theupper potion of the vent structure and a number of small bleed holes inthe cover structure. The adapter plug is shaped to be suitably attachedto the upper end of a control pin, and the baffle allows free passage ofgas while preventing the passage of particulate matter and impeding thepassage of molten sodium. The possibility of sodium backflow isvirtually eliminated by use of the air lock and the anti-splash porousmetal baffle.

BRIEF DESCRIPTION OF THE DRAWINGS My invention will be more fullyunderstood, and other advantages and features thereof will becomeapparent, from the following description of an exemplary embodiment andmethod of the invention. The description is to be taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 is an elevational view, shown in section, of a vent assemblywhich is to be affixed to the normally upper end of a reactor controlpin;

FIG. 2 is a fragmentary elevational view, shown in section and enlarged,of a vent structure constructed according to this invention; and

FIGS. 3A, 3B, 3C, 3D, 3E, 3F, 3G, 3H, 3I and 3.] illustrate certain mainsteps in a method of fabricating the vent structure.

DESCRIPTION OF THE PRESENT EMBODIMENT AND METHOD In the followingdescription and accompanying drawings of an illustrative embodiment andmethod of my invention. some specific dimensions and types of materialsare disclosed. It is to be understood, of course, that such dimensionsand types of materials are given as examples only and are not intendedto limit the scope of this invention in any manner.

FIG. 1 is an elevational view, shown in section, of the upper ventassembly of a control pin 12 which is used, for example, in a liquidmetal (sodium) cooled fast breeder nuclear reactor (not shown). Thecontrol pin 12 and its upper vent assembly 10 are normally fullyimmersed deeply in (under approximately 8 feet of) the molten sodiumreactor coolant operating'at temperatures of about 900 to 1,100F. Eachcontrol pin 12 contains a series of boron carbide pellets (not shown)and is a source of generated helium. The vent assembly 10 is, of course,hermetically attached to the upper end of the control pin 12 andgenerally includes a vent adapter plug 14, baffle 16, air lock tube 18,air lock cap 20, and vent structure 22.

The vent plug 14, air lock tube 18, and air lock cap are preferably madeof a material similar to that of the control pin 12 structural materialsuch as Type 316 stainless steel, which is compatible with a hot sodium(liquid and vapor) environment. Baffle 16 is made of a porous metalwhich is resistant to attack by hot sodium. A felt or foam metal can beused and, in the exemplary vent assembly 10, a Type F-315 nickel feltmetal produced by Huyck Metals Corporation was used. This material isabout 20 percent dense and allows free passage of gas while preventingthe passage of particulate matter and impeding any passage of moltensodium.

The air lock tube 18 has, for example, four vent or bleed holes 24 whichcan be 0.0135 inch in diameter equiangularly spaced 90 about the airlock tube at a predetermined distance above the lower end thereof. Thevent plug 14 has an axially drilled central hole 26 and the baffle 16also has an axially punched central hole 28. The punched baffle 16 ispacked on the vent structure 22 against the upper surface 30 of ventplug 14 which is joined at its lower end 32 to the lower end of the ventstructure by a standing lip electron-beam weld for maximum cleanlinessand minimal disturbance of the surrounding metal.

When the lower end of the air lock tube 18 is welded to flange 34 of thevent plug 14, the vent holes 24 are located slightly above the uppersurface 30 of the vent plug and directly adjacent to the lower sidesurface of the baffle 16. Welding of the air lock cap 20 to the upperend of the air lock tube 18 forms an air lock chamber 36 containingcover gas which provides an air lock effect over the vent structure 22such that liquid sodium does not directly or normally contact the uppervent end 38. The possibility of sodium backflow is virtually eliminatedby use of the air lock chamber 36 and the anti-splash porous metalbaffle 16 in the vent assembly 10 of control pin 12.

The air lock chamber 36 is a plenum chamber which is made large enoughto provide a sufficient reservoir of gas that prevents liquid sodiumwhich might enter the lower bleed holes 24 and into the baffle 16, as inthe event of any sudden fluctuation (loss) in gas pressure due to atemporary reactor temperature change (drop), from ever reaching the ventopening in the upper vent end 38 of the vent structure 22. Of course.the sodium is subsequenntly forced out of the chamber 36 following thetemperature change as the gas pressure therein builds up to equalizewith the environmental (8 feet of liquid sodium) pressure.

The bleed holes 24 are made adequately small so that they will not admitthe surge of liquid sodium into the air lock chamber 36, which surge canoccur when the control pin 12 is first immersed in the sodium. On theother hand, the bleed holes 24 are made adequately large mainly forconvenience of drilling very small holes in stainless steel with thepresently available drills and methods. In any event, the combined sizeof the bleed holes 24 must be about equal or (and are vastly) largerthan the effective size of the vent discharge opening in the upper ventend 38 of the vent structure 22, and allows gas to escape from thechamber 36 at about the rate that it is being released from the ventstructure.

The vent assembly 10 has general overall dimensions of length A,diameter B, and an approximate gas space length C (connected throughvent holes 24 to the exterior). Illustrativevalues for these dimensionsare A of 1.80 inches, B of 0.435 inch (maximum), and C of 0.95 inch, forexample. This vent assembly 10 is, of course, to be welded to the upperend of the stainless steel tube (control pin 12) having a 0.395 inchinside diameter which accommodates the lower portion of the vent plug14. Other dimensions of the vent assembly 10 can be proportionatelyestimated adequately from the elevational'view of FIG. 1 and, whileapproximate, will suflice for most purposes. The vent has a gas flowrate sufficient to maintain pressure in the air lock chamber 36 and incontrol pin 12 to prevent backflow of sodium and any possiblycontaminating outside cover gas, respectively.

FIG. 2 is a fragmentary elevational view, shown somewhat enlarged insection, of the vent structure 22. The vent structure 22 inclues a thinwall tubing 40 having a undeformed lower portion 42, and a flattened andfolded and pressed upper portion 44. The flattened upper portion 44 ispreferably folded at least twice, as shown. A particulate filter 46 isprovided in the lower portion 42 of the tubing 40, and a film or layer48 of fine particles or powder is provided between the pressed opposingsurfaces inside the upper portion 44. The tubing 40 can be made of anymaterial sufficiently ductile to survive the flattening, folding andpressing operations without cracking, and there are appropriatematerials readily available for operation or use under a very wide rangeof environmental and temperature conditions.

In the interests of compatibility and uniformity, the tubing 40 can bemade of the same material as the control pin 12 structural material.Thus, the tubing 40 can be made of Type 316 stainless steel having aninside diameter D and an outside diameter E as indicated in FIG. 2.Likewise, the particulate filter 46 can be made of Type F-315 nickelfelt metal similar to that of baffle 16 (FIG. 1) and having a length F.The cylindrically shaped filter 46 can be installed in the lower portion42 by press-fitting it into the tubing 40 to a position approximately asillustrated. Exemplary dimensions for the inside diameter D, outsidediameter E, and length F are respectively 0.093, 0.125, and 0.250 inch,for example.

The formed ,upper portion 44 of the exemplary vent structure 22 has alength G and a thickness [-1 as indicated in FIG. 2. The film or layer48 can consist essentially of, for example, compacted 0.3 microndiameter aluminum oxide (A1 0 particles. This particle size could beincreased to as large as 1 micron or reduced to a small as 0.05 micronfor entirely satisfactory operation. A particle size larger than 1micron might provide individual support points during forming(flattening and pressing of the upper portion 44 and which may preventthe attainment of very low gas flow rates. Particles sizes larger than 1micron are nevertheless desirably and effectively used satisfactorily inthe vent structure 22. Of course, a particle size smaller than 0.05micron may not prevent eventual grain growth across the proximate facesof a flattened tube athigh temperatures. Thus, in regular long term andhigh temperature operation, it is possible for the formed upper portion44 of .the vent structure to sinter together.

Illustrative dimensions for the length G and thickness H arerespectively about 0.350 and 0.092 inch, for example. The thickness ofthe film or layer 48 can be approximately 0.005 inch, when pressed,although the layer thickness in the exemplary vent structure 22 can begenerally between 0.003 and 0.007 inch for satisfactory operation. Exactpressing parameters and anneal conditions are functions of theparticular size and wall thickness of the tubing 40 used and the desiredgas flow rate. It may be noted that the formed vent structure 22, withor without the layer 48 of alumina particles, can be used independentlyas a gas flow metering device in various applications following suitableflow calibratio thereof.

A substantially linear relationship exists in the response in heliumflow rate of the vent structure 22 to change in helium pressure, andhelium flow rate drops linearly as the driving pressure decreases. Flowrate is, of course, lower at the higher temperatures since there areless gas molecules in a fixed volume at such temperatures with themaintained pressure differential. With the helium generation rate percontrol pin 12 (FIG. 1) at approximately 3 cc/hr, for example, theventstructure 22 provides an average helium diffusion rate of about 1cc/hr at one atmosphere pressure differential after about 60 thermalcycles. This allows a helium pressure of approximately 4 atmospheresinside the control pin 12 to provide a two-fold advantage. First,pressurized helium has a higher thermal conductivity to dissipateradiation heating in the controlpin and, second, the pressure minimizesany possibility of back diffusion of either sodium vapor or cover gasinto the control pin. The positive helium pressure maintained in thecontrol pin 12 by the vent structure 22 can drop by a factor of about 2without compromising the sodium seal provided.

FIGS. 3A through 3.] illustrate certain main steps in the method offabricating the vent structure 22. In FIG. 3A, a Type 316 stainlesssteel tubing 40 of 0.125 OD. and 0.016 inch wall thickness is cut to apredetermined length of 10 inches, for example. A slight chamfer 40a asshown in FIG. 3B is applied to the inside edge of the end to be pressedas a precaution because metallographic inspection has shown a tendencyfor the mouth of the vent to be indented during pressing. The tubing 40is then vacuum annealed 15 minutes at l,900F, for example, andultrasonically inspected for any surface or subsurface flaws. Austeniticstainless steel is virtually unaffected by pure hot sodium attemperatures below 1,000F. In the region between l,000 and 1,500F,however, there is evidence of some measurable attack, particularly atthe grain boundaries. Based on this constraint, the vent tubing 40 wallthickness was selected to be 0.016 inch to provide a safety factor of atleast 3 over the deepest sodium penetration observed at 1,500F.

The annealed tubing 40 is next placed over a control rod or shaft Smounted upright in a resilient block y with the chamfered tubing enddown as illustrated in FIG. 3C. The rod 5 can be a carefully polishedbrass rod 1.5 inches long and 0.065 inch in diameter, and the resilientblock y can be of rubber, for example. The tubing 40 may be manuallyheld concentrically about the rod S against the block y and anappropriate surface of block y for a distance equal to at least thelength of the upper tubing portion 44. The alumina particles are thenpacked down around the control rod S using, for example, a hand heldvibrator K positioned with its output shaft p in contact with the sideof the tubing 40 as shown in FIG. 3D. The vibrator K can be, forexample, a commonly available electric scriber which has an output shaftthat reciprocates axially in and out at 7,200 cps. The stroke length onsuch a scriber can be adjusted (for different diameter tubings 40) by aknob (not shown) at the top of the tool. An even yet thin layer ofparticulate ceramic can thus be obtained for an effective grain growthbarrier. By varying the diameter of the control rod S, any requiredthickness of ceramic layer can be provided. It should be noted that thecontrol rod S functions such that it is not critical that the tubing 40need be held very concentrically about the control rod.

The assembly shown in FIG. 3D is turned horizontal and the control rod Sis withdrawn to leave a tube of alumina within the tubing 40 as depictedin FIG. 3E. This partially coated tubing 40 is then transferred to, forexample, a Carver hydraulic press for subsequent flattening and pressingoperations. A predetermined length of the internally coated end portionof the tubing 40 is flattened in the Carver press at a predeterminedflattening pressure. The ceramic layered end portion of the tubing 40can be placed on a tool steel block T1 approximately 1 inch wide so thatthe chamfered end just barely overhangs and a second block T2 is placedon top as shown in FIG. 3F. The blocks T1 and T2 are suitably positionedin the Carver press which is operated at a predetermined pressure of,for example, 2,000 psi and released to produce a flattened area asillustrated in FIG. 3G. The flattened portion of tubing 40 can be nextformed by manually bending it (with a pair of pliers) into a Zconfiguration as shown in FIG. 3H, and a final pressing operation isperformed thereon in the Carver press at a predetermined pressingpressure of, for example, 2,000 psi. This produces the configurationillustrated in FIGS. 31 and 3] and which creates a flat tortuous pathfor the escaping gas.

The compacted alumina throughout the vent wall interface eliminates theoperational sintering tendency due to grain growth across the faces ofthe flattened tubing 40 at high temperatures. The formed tubing 40 isthen preferably vacuum annealed minutes at 1,900 "P again. A particulatefilter 46 is finally presstitted into the formed tubing 40 as indicatedin FIG. 3.] to complete the vent structure 22. The formed tubing 40 ispreferably annealed particularly where the vent structure 22 is used inhigh temperature (600C or 1,112F) operation because fiow rate doubles inthe vent structure going from a stressed to annealed condition andoperating at the high temperature of 600C, the vent will self-annealover a period of 3 weeks.

The alumina or aluminum oxide powder is compatible with any vent tubingmaterial and is uniquely suited for operation in the presence of hotsodium vapor. It is an extremely stable oxide and is the only commonlyprocessed oxide resistant to attack by hot sodium and sodium vapor. Asmentioned previously, however, exact pressing parameters (and annealconditions) are a function of the particular size and wall thickness ofthe vent tubing used and the desired gas flow rate. In the flatteningand pressing operations required on the particular vent tubing 40 of0.125 inch OD. and 0.016 inch wall, gasflow rate at 15 psi differentialpressure is illustratively varied according to flattening and pressingpressures as indicated below.

2) of tubing can include the particulate filter 46 which allows freepassage of gas. To produce the particulate filter 46, nickel felt metalcan be used. Of course, a foamed instead of felt metal can be used andthe metal can be stainless steel instead of nickel, for example, sincesuch materials are resistant to attack by hot sodium and allow freepassage of gas. A filter disc or discs of appropriate size can bepunched out from the nickel felt metal to form the filter 46.

metal. This baffle material can, of course, be nickel instead ofstainless steel and felt instead of foam metal. Indeed, it is usuallyconvenient and preferable that the baffle 16 and filter 46 be made ofthe same identical material. The foam metal is punched or otherwiseshaped to size and configuration. In similar manner, the vent adapterplug 14, air lock tube 18 and air lock cap 20 can be shaped from a Type316 stainless steel rod. The plug 14, tube 18 and cap 20 can be machinedto size and shape from the stainless steel rod. It is, of course,apparent that the formed tubing 40, filter 46, baffle 16, adapter plug14, air lock tube 18 and air lock cap 20 can be fabricated in anyelected sequence or all concurrently.

Flattening Pressure Pressing lressure (P For 0.3 micron average aluminaparticle size:

1000 2000 2000 4000 4000 For 0.1 micron average alumina particle size:

9. 9 N OKIIUILII N Flow Rate (cc/hr at 15 psi A P) By changes inpressing pressures and techniques, the vent structure 22 can befabricated for any required flow down to, for example, 0.0036 cc/hrhelium at one atmosphere pressure differential. Also, flow through thevent structure 22 can increase greatly-in case of a sudden increase inhelium pressure within the control pin 12. Thus, the vent structure 22effectively acts to relieve pressure transients in the control pin 12 toprevent any possible ruptures thereof and then returns by naturalspringback to normal operation if the elastic limit of the formed tubing40 has not been exceeded. The illustrative vent structure 22 has beentested and found good to over 3,500 psi, for example. While the doublyfolded, formed tubing 40 does not actually unfold in order to relievehigh increases in pressure, it tends to do so. This was substantiated intesting a vent structure 22 with increasing pressure until it burst at avery high pressure, when the formed tubing 40 did unfold to some extent.

Because of the production oflithium as a result of the (neutron, alpha)reaction on boron carbide and the possibility of boron carbidedisintegration under irradiation, the unpressed section (lower portion42 in FIG.

The particulate filter 46 is packed into the formed surface of thebaffle l6 and is joined at its lower end 32 to the lower end of the ventstructure by a standing lip electron-beam weld. A helium leak check ismade of the electron-beam weld, and the air lock cap 20 istungsten-inert-gas (TIG) or electron-beam welded to the air lock tube 18with its vent holes 24 (FIG. 1) located at the opposite end thereof awayfrom the cap. The air lock tube 18 is then TIG welded to the adapterplug 14 to complete vent assembly 10. Completed assemblies can be packedin plastic bags and sealed for delivery.

The vent structure 22 is, for example, produced for operation in ahostile environment including high purity molten sodium at 600C and aradiation fluence level of 10 to 10 nvt (neutron density-velocity-timeor neutrons/cm with an average neutron energy of 70.1 Mev (millionelectron volts). It is, however, evident that the vent structure 22 canbe readily adapted for use to any application where a controlled gasrelease in either a friendly or hostile environment is necessary. Theonly requirement is choice of metal tubing 9 10 and ceramic compatiblewith the environment and caincluding an undeformed normally lowerportion pableof fabrication into the desired shape. and a formednormally upper portion, and a partic- Whlle exemplary embodiment andmethod of this ulate filter installed in said lower tubing portion,

invention has been described above and shown in the accompanyingdrawings, it is to be understood that 5 such embodiment and method aremerely illustrative of, and not restrictive on, the broad invention andthat I do not desire to be limited in my invention to the speand saidupper tubing portion being flattened and folded against itself at leasttwice in a generally Z configuration and a relatively thin layer ofsmall sized parcific constructions or arrangements or steps shown andtlckfs m Sa1d upper tubmg porno Separatmg 9 described, for variousobvious modifications may occur 10 posmg proximat faces a capable ofPrevemmg to persons having ordinary skill in the art. themlfrom Smtermgtogether- I claim: 2. The invention as defined in claim 1 wherein the 1.A vent structure comprising: end of said upper tubing portion ischamfered. a tubing of predetermined length and size, said tubing

1. A vent structure comprising: a tubing of predetermined length andsize, said tubing including an undeformed normally lower portion and aformed normally upper portion, and a particulate filter installed insaid lower tubing portion, and said upper tubing portion being flattenedand folded against itself at least twice in a generally Z configurationand a relatively thin layer of small sized particles in said uppertubing portion separating its opposing proximate faces and capable ofpreventing them from sintering together.
 2. The invention as defined inclaim 1 wherein the end of said upper tubing portion is chamfered.